Studies were performed on mongrel dogs to determine the effects of the purine nucleoside inosine on the in situ heart. Intravenous infusion of 5-50 mM inosine produced arterial blood concentrations of 20-100 microns. These infusions of inosine were associated with progressive increases in the left coronary blood flow. The maximum increase in contractile force observed with 50mM inosine averaged approximately 45%, and the mean increase in coronary blood flow averaged approximately 100%. The inotropic and the coronary blood flow effects of inosine were not altered by beta-adrenergic blockade, indicating that the effects were not mediated through adrenergic mechanisms. In additional studies it was demonstrated that following coronary occlusion, inosine infusion increased the collateral perfusion of the ischemic muscle as determined with tracer microspheres. This increase in collateral blood flow to ischemic muscle was associated with a substantial reduction in infarct size. In progress are studies evaluating the effects of other nucleobases or nucleosides on the in situ canine heart. Specifically, the nucleobase hypanthine is being investigated.